The present invention generally relates to the field of sensors, and more particularly to an apparatus and method for electromagnetic sensing of fluid flow.
A magnetic or electromagnetic liquid flow meter is commonly used for determining the flow rate or volumetric flow of a conductive liquid or media, such as fresh water or wastewater, flowing within a conduit. A magnetic flow meter operates in accordance with Faraday's Law stating that a moving conductor in a magnetic field generates a voltage, the amplitude of which is proportional to the velocity of the conductor. For a magnetic flow meter, the moving conductor is the liquid or media flowing adjacent the flow meter.
A conventional electromagnetic flow meter will now be described with reference to FIG. 1. This flow meter include a pair of electrodes 10 and 12 located at diametrically opposite positions across the opening of a flow tube 14. Typically, the electrodes are insulated from the flow tube by a liner 16. As shown, electrodes 10 and 12 are fixed on an axis at right angles to the axis of a pair of exciter coils 18 and 20. Exciter coils 18 and 20 are each fixed at diametrically opposite positions on the outside of flow tube 14.
Such conventional electromagnetic flow meters are calibrated and then used to measure flow passing through the flow tube. Occasionally, it is desirable to revalidate the calibration in order to ensure accuracy of the flow meter. It is normally not practical, however, to hydraulically revalidate the calibration on site. It is also not practical to withdraw the sensing embodiment from such typical systems to hydraulically check and revalidate the sensor on site using a hydraulic calibrator. Consequently, revalidation of typical electromagnetic flow meters is limited to electronic or electrical checks at a remote transmitter, which reveals only the state of the flow meter electronics and coils, without addressing mechanical movements that could have occurred and affected the hydraulic condition of the flow meter.
Ultrasonic time of flight meters may be used for calibration at a job site. Such meters are normally installed temporarily by being strapped to the outside of the pipe. They utilize the principle that sound waves emitted from outside the pipe will propagate through the media to a receiver and back again. The propagation is directly proportional to the mean velocity of that portion of the media in which the sound waves are directed. At zero flow, the sound waves reach both the emitter and the receiver and back again at the same time, without transit time delay. With flowing media, the sound waves take differing lengths of time between emitter and receiver, dependant on the media velocity and whether the sound waves travel against the flow stream or with the flow stream.
Accuracy of this type of flow meter is adversely affected if the sound waves are directed at only one thin section of the media velocity. Consequently, the mean velocity of the complete pipe cross section may not be measured. This causes significant errors if straight lengths of the pipe structure are relatively short, particularly as required of a magnetic flow meter.
Other inaccuracies result from the ultrasonic signal being affected by encrustation or other media coatings inside the pipe. It is also not always possible to install such a flow meter in the relatively long length of pipe necessary. Consequently, accuracy on site can be typically 1-10% of rate (i.e., the measured value may deviate 1-10% from actual value). That is insufficient for the required revalidated 0.5% accuracy required of a magmeter in the market.
Calibration has also been attempted using tracer techniques, typically involving transit time or dilution methods. A common tracer method operates on the principle of adding a tracer, such as sodium chloride (using conductivity probes) or potassium iodine (spectrophotometric probes), at a known point in the pipeline. The presence of the tracer is then detected at a known distance (volume) at another part of the pipeline. Volumetric flow can be inferred from the timing of such detection.
Dilution methods typically involve injecting the tracer at a fixed rate and concentration in the pipe structure. Its concentration in the media rises until it reaches a constant amount. When this occurs, water samples, by way of example, can be removed and the flow rate calculated from the injection volume, the undiluted tracer concentration at the point of injection and concentration at the point of constant dilution.
Problems with such dilution methods include undesirable time consumption, expensive work, need for skilled personnel and complex analysis methods. For example, mixing errors may introduce inaccuracies into the measurement. Accuracy is also dependant on internal sample pipe coatings and the distance between injection and sampling. In addition, lower velocities require longer injection and sampling distances. Moreover, the flow has to be turned off to install the equipment rendering such methods impractical in many situations.
Insertion master flow meters have also been utilized to calibrate electromagnetic flow meters. In this regard, a point velocity is typically measured from which the mean velocity is derived. This is often accomplished by traversing the insertion probe and taking an average of the velocity profile from which volumetric flow is inferred.
One problem with an insertion master flow meter is the excessive time which is often required. The probe must be installed via an isolating valve and pressure chamber. A hole is drilled in the pipe structure by a special under pressure drilling machine, unless the flow can be turned off.
Insertion master meters are typically turbine meters or vortex meters, which generally have accuracy of 1-4% depending on the specific knowledge of pipe diameter and obstruction factors. If an insertion “mag” meter is utilized, the accuracy may be in the range of 2-5% of measured rate of flow, which is insufficient for revalidation of magmeters. The reasons for this are low magnetic flux and very close electrode spacing. Again, this is greater than the revalidated 0.5% accuracy required of magmeters in the market.
These well known methods typically require the diameter of the sample pipe to be accurately known, since the error in volumetric flow is proportional to the square of the mean pipe diameter error. All these techniques and others are used to calibrate electromagnetic flow meters on-site by media passing through the pipeline. As such, they require minimum straight length of pipe typically from 10-20 diameters upstream of point of sensing or sampling. Such long pipe length requirements are often not practical, are not desirable in their use as a portable calibrator, nor are such straight lengths often available on site.